Optical systems in the form of microlithographic projection exposure apparatuses serve for producing microstructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit or a projection lens. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The shorter the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Imaging light having the wavelength of 193 nm or imaging light having a wavelength in the range of the extreme ultraviolet (EUV), i.e. 5 nm-30 nm, is primarily used nowadays. When imaging light having a wavelength of 193 nm is used, both refractive optical elements and reflective optical elements are used within the microlithographic projection exposure apparatus. When imaging light having a wavelength in the range of 5 nm-30 nm is used, by contrast, exclusively reflective optical elements (mirrors for EUV lithography) are used.
The performance of an optical system, e.g. of a projection lens, comprising mirrors (in particular, but not exclusively, in EUV systems) is largely determined by the deviations between the system wavefront defined in the design and the system wavefront achieved in production. These deviations arise (alongside the contributions of mounting and alignment) primarily during the production of the individual components (mirrors) as a result of the accuracy of the optical production, of the coating and of the corresponding measurement techniques. If the stipulations regarding the accuracy of an individual mirror component or, as a result of the interaction of a plurality of defective mirror components, the specified wavefront sizes are not achieved in production, one or more mirrors must be exchanged or reworked until the overall system fulfills its specifications.
Various approaches are known for correcting wavefront contributions of individual (mirror) components or of an overall optical system:
WO 97/33203 describes an imaging optical system for UV radiation, wherein a reflective surface is provided with a correction layer which is transparent to radiation at the used wavelength and which is intended to compensate for unevennesses in the surface form of the reflective surface by means of a layer thickness variation in order to produce a correction of the wavefront in this way.
EP 1947682 describes a correction process for a mirror with a multilayer coating, wherein, by means of ion beam figuring (IBF) for example, a removal is effected in the topmost layers of the multilayer coating in order to produce a thickness distribution and to alter the wavefront. An intermediate layer composed of Si or an Si-containing material is applied to the part of the multilayer coating subjected to removal, and a protective layer having a substantially constant thickness is applied to the planar top side of the intermediate layer.
Although aberrations are corrected in both of the correction processes described above the reflectivity of the mirrors will vary greatly as a result of the processing near the surface, as a result of which the apodization is impaired. Moreover, the figuring of different layer materials (Mo and Si) e.g. by means of an ion beam as described in EP 1947682 constitutes a problem in terms of processing engineering: during the figuring, different removals and roughnesses of the layer materials result therefrom. In particular, the figuring in a molybdenum layer is typically disadvantageous since extensive roughening and oxidation generally occur in said layer.
US 2008/0259439 A1 describes the correction of the wavefront of a mirror by subjecting the layers of the reflective coating to radiation, in particular to laser radiation, for heating the layers. This makes use of the fact that the heating results in compaction of the layers, which leads to a reduction of the period length. The compaction of the layers results in a shift in the reflectivity spectra in the irradiated regions, as a result of which the apodization is likewise impaired.
US 2007/0091420 A1 and US 2007/0091421 A1 describe mirrors comprising a multilayer coating, wherein an intermediate layer composed of Si is arranged between a first group of layers and a second group of layers. A sputtering process is proposed for applying the layers, in which process the sputtered particles impinge on the substrate perpendicularly or at an angle.
DE 10 2009 029471 A1 describes a mirror comprising a substrate and a reflective coating. The reflective coating comprises a first group and a second group of layers, wherein the second group of layers is arranged between the first group of layers and the substrate. The layers of the first and second groups serve in each case for reflecting EUV radiation. The first group of layers comprises a number of layers that is greater than 20 such that, upon irradiation with radiation having a wavelength in the range of 5-30 nm, less than 20% of the radiation reaches the second group of layers. The second group of layers has a layer thickness variation for correcting the surface form of the mirror. In one exemplary embodiment, the second group of layers has a correction layer having a layer thickness variation which makes up more than 50% of the layer thickness variation of the second group of layers. The layer thickness variation in the correction layer can be produced by ion beam figuring for example. Since only approximately 20% of the incident radiation reaches the second group of layers, the combination of both groups of layers is intended not to have a significant effect on the reflectivity properties of the mirror.